Making Rain Forecasts More Accurate

Rainfall forecasts are flawed. They don’t take into account factors such as evaporation making them less accurate. Meteorologists at the University of Missouri (MU) have developed a method to help accurately account for evaporation and deliver an accurate rain forecast.

Forecasts are especially inaccurate for people living 30 miles or more away from National Weather Service radar stations. Radar beams rise higher into the atmosphere as they travel, which means it’s not accounting for what happens to the drop after it’s detected.

“Many of the areas that are further [stet] from the radar have a lot of agriculture,” says Neil Fox, associate professor of atmospheric science in the School of Natural Resources at MU. “Farmers depend on rainfall estimates to help them manage their crops, so the more accurate we can make forecasts, the more those forecasts can benefit the people who rely on them.”

MU’s research measures the impact evaporation has on the amount of rainfall that actually reaches the ground. They use dual-polarization radar, which sends two radar beams polarized vertically and horizontally that differentiate the sizes of drops. Size effects evaporation rate and its motion—smaller raindrops evaporate faster but encounter less air resistance.

The team combined this information with details about humidity in the atmosphere to follow raindrops from when they’re observed by radar, to when they hit the ground to predict how much evaporation occurs for each drop. This improves actual rainfall estimates.